Hi folks, first time poster here. Zone 6. I’ve gone through these archives fairly extensively looking for answers, but they seem sort of bottomless, so sorry if I’ve missed something, and here goes:

I’m updating a 1950 cape cod 1 1/2 story that is just a 28’x28′ box with a simple gable roof and small gable roof over the entry door. I’ll be re-siding, replacing a few windows, installing a patio door, building out the porch roof, and insulating it this summer. I’m hoping to add exterior insulation, do it as “green” as possible and detail it as robustly as possible within a meager budget. I’m not going for broke because frankly we’ve opted to stay in our little house vs. move for love of the neighborhood, and it’s fairly efficient in terms of heating, cooling, and resource use already because of it’s size. SO….

I’ll be ripping off old, insulated vinyl, finding the source of a mold issue, then re-siding it. My dirt cheap insulating/ siding assembly will be, from inside to out:

EXISTING:
1. Several layers of paint 
2. 1/2″ plaster
3. 3/8″ gypsum board
4. 2×4 wall with old fiberglass batts
5. 3/4″ sheathing boards
6. felt paper
7. Original 3/4″ dutch lap (?) siding

NEW:
8. Tyvek, fully taped
9. 2 layers of 1″ EPS foam, seams staggered & taped
10. furring strips for rain screen
11. New siding, be it fiber cement, LP, or discounted, old warehouse vinyl currently available for $25/ square
12. Windows “innie” style with extension jambs, unless I end up pulling them all to replace, in which case window bucks and “outie” style

This house is never going to be a beauty, but it can look and behave a lot better than it does now, and that’s my goal. My thoughts with this assembly are:

(a) Tyvek is good enough
(b) 2 layers of 1″ EPS should be R-8 and exceed the guideline of R-7.5 for 2×4 walls in zone 6
(c) EPS is the most vapor permeable and greenest of the foam options, and Roxul Comfortboard 80 won’t get me to R-7.5
(d) I can’t do anything about the insulation inside my walls without opening them up because there’s already old (bad) fiberglass batts inside, so exterior is the way to go

There is the problem of the gable end — I’ll need to somehow build out the rakes at the gable ends to cover the tops of the foam. They’re currently fairly flush with the side of the house. How to do this cheaply and without replacing the roof, which has a few more years in it? 

There is also the potential problem of cheap EPS being pretty un-resilient inside a rainscreen in terms of crumbling, etc. Would I want to cover it in another layer of housewrap? Or just move the tyvek to the exterior?

Oh, and I’ll be exposing the foundation wall, which is currently covered in the insulated vinyl down to ground level. I want it to be able to breathe to the exterior since I’ll be insulating the basement with InSoFast EPS panels this winter. 

Thanks so much in advance for the aid of this brain trust!

~Matt